The present invention relates to a novel process for the treatment of aqueous streams such as waste streams and metal ion-containing or other precipitatable material aqueous streams with a minimum of treatment water by utilizing reverse osmosis and electrodialytic water splitting, in sequence.
At the present time, given the ever-increasing cost of energy and the increased demands for the control of environmental pollution on the one hand, and the need to conserve important metal resources on the other, there arises a serious need for new and cost-effective processes to treat industrial process streams some of which are dilute in metals. Sometimes this is done for the purpose of recovering metal values; other times it is for the purpose of avoiding environmental pollution.
Typical metals which are sources of environmental pollution and which are valuable as minerals are copper, lead and cadmium, which are found in a wide range of waste waters resulting from plating and finishing processes such as metal finishing, from various rinses from pickling operations, from acid mine drainage and also from the extraction of metals as from depleted ore tailings by dump leaching. In certain processes, the rinse solutions are so dilute that a simple process of neutralization, precipitation and collection of an insoluble precipitate suffices. However, in the case of cadmium and lead as examples, the allowable toxic metal levels in effluents are so low that removal by precipitation often does not suffice. In the case of the copper recovery from the acid leach dumps, the residual concentration of acid is so high as to require a substantial amount of base for its neutralization.
In many industrial processes which formerly were able to employ lime for the precipitation of heavy metals and for the neutralization of acid such as sulfuric acid, with the formation of insoluble calcium sulfate, these are no longer environmentally acceptable because even the low level of contamination of heavy metals causes the solid waste to be classified as a toxic waste, requiring transportation to special dumps at considerable expense.
Electrodialysis, including especially electrodialytic water-splitting, processes are well known and of considerable value in industry. In electrodialysis, electrolytes are removed by the electric current from a feed compartment and selectively transported across ion-permeable membranes into a concentrate stream, with the feed stream being thus converted into a deionate stream. In water splitting, a feed stream contains anions and cations, wherein the anions of the feed stream plus hydrogen ions from the water pass by the current from a bipolar membrane into an acid compartment, while cations from the feed stream plus hydroxide ions from the water pass from the bipolar membrane into a base compartment.
The electrodialysis process is highly developed and is described in several texts including the chapter by Shaffer and Mintz in "Principles of Desalination, Part A" edited by Spigler and Laird, Academic Press, New York 1980. Bipolar membranes and the process of water splitting are also well known. Bipolar membranes have been articles of commerce for several years, with those described by Dege, Chlanda et al., U.S. Pat. Nos. 4,116,889 and 4,253,900 as recent examples. Water-splitting processes are also described in several recent patents including U.S. Pat. Nos. 3,787,304, 4,219,396, 4,238,305 and 4,017,015.
Likewise, the technique of reverse osmosis (hereinafter "RO") is well known and has been utilized for various applications. For example, RO has been employed in the automobile industry to treat the rinse water emanating from various painting operations.
Yet, these technologies have not been heretofore utilized cojointly in a fashion to deal with the several and diverse problems associated with the problem streams described herein.
It is accordingly an object of the invention to provide a process which will handle all these manifold problem streams, one which does not require elaborate chemical processing and one which can be operated on the very large scale of acid leach operations on the one hand as for copper recovery and also on a small scale for battery manufacture, smelters, electroplating, metal finishing and pickling operations on the other.
Another general object of this invention provides a process which can eliminate the disposal problems associated with toxic wastes. A related and more specific object is to provide a process in which the toxic wastes that would otherwise have to be disposed of are rendered useful so as to be capable of being returned in some fashion to either the manufacturing process involved or as an aid in the treatment of the stream resulting from such manufacturing process.